Nonwoven materials are commonly used in absorptive devices, such as diapers and feminine napkins. Nonwoven materials are often used as topsheet components of such absorptive devices where it is desirable to achieve softness due to the contact of the topsheet with the skin of the wearer of the absorptive device. A ratio of high loft to total thickness of the nonwoven material often indicates softness, because the nonwoven material is compressible, which in turn renders a softness sensation to the wearer of the absorptive device that includes the nonwoven material. While relatively high loft nonwovens are perceived to be soft and cool when used against the skin, special processing is typically needed to achieve such characteristics, which may increase the cost of the product.
Fiber entanglement by injecting and withdrawing barbed needles is one known method for creating relatively high loft nonwoven materials, but the process is relatively slow and costly. A faster production method for generating high loft nonwoven materials is a spunlacing process for hydro-entanglement of loose fibers. The spunlacing process may create relatively high lofted soft nonwoven materials that are soft and cool to the touch by using high pressure water jets that are essentially in the shape and diameter of needles to hydro-entangle the fibers.
Spunlacing is a process of entangling a web of loose fibers on a porous belt or a moving perforated or patterned screen to form a sheet structure by subjecting the fibers to multiple rows of fine high-pressure jets of water. The spunlacing process uses an array of very fine high velocity water jets, essentially the diameters of needles, instead of barbed needles, to entangle the fibers in order gain web integrity while yielding a relatively high loft nonwoven material. The needle-like water jets are applied by a high pressure header, and the pressure may range from 2000 psi to over 8000 psi. The water needle jet holes are typically about 0.005 inch in diameter and a single header may include between about 30 and about 80 holes per inch in a row. Three to eight headers may be placed in a row that is aligned in the machine direction, and the web of untangled fibers may move in the machine direction on a perforated belt or screen-like material. A vacuum zone exists underneath the belt to extract the water. After the fibers are hydro-entangled into a web, the web may be dried and wound into a roll that can then be unwound when converted and used as a layer in an absorptive device. The spunlacing process generally does not lend itself to produce laminates with a film layer while the lofting is occurring because the high pressure and needle-like shape of the water jets may damage the film layer, and may possibly remove most or all of the film from the nonwoven layer.
Spun bonded nonwovens are less costly than spunlace nonwovens, but typically have much less loft and are often not as soft as other nonwoven materials. The spun bonded process for making a nonwoven web is known. In a so-called Reicofil system, polymer pellets are fed into an extruder that extrudes continuous fibers through a die with a plurality of small openings. The fibers are thinned or stretched and cooled as the polymer exits the die. The fibers are then spun to random positions by air currents provided by manifolds or other devices. After the fibers are spun, the fibers are randomly positioned on a moving belt made of open screen material to create a matt of spun fibers. Suction may be applied to ensure the entangled fibers lay flat in a substantially horizontal orientation and are essentially pinned on the moving screen.
The matt of entangled fibers may then be fed into a calendar roll nip, with one roll having a smooth surface and one roll having raised points in a pattern. Both rolls may be heated to a point above the melting point of the polymer in the fibers. The matt is compressed as the raised points compress the matt against the smooth roll. The heat and pressure applied to the matt creates bonded points that hold the fibers in place to create a spun bonded nonwoven web.
Nonwoven materials, whether created by spun bonded, air laid, carded, spun laced, hydro-entangled, or other processes, have a basis weight that defines the mass of the fibers contained therein (typically measured in grams) within a square area (typically measured in a square meter) so that the basis weight is measured in grams per square meter (“gsm”). In addition, all fibers have a thickness or diameter that is referred to as denier. A nonwoven material having fibers with a heavier denier and fewer fibers can have the same basis weight as a nonwoven material having fibers with a lighter, or thinner denier and many more fibers. Features such as loft (thickness), which is a distance measured from the top of the nonwoven web to the bottom of the nonwoven web, for a given mass of fibers may be manipulated by choosing a fiber denier and process technique for creating loft, while entangling or bonding the fiber web so the web will have tensile strength and web integrity. Unfortunately, the spun bonded process does not lend itself to producing higher lofted nonwovens due its tendency to have horizontal fibers.
The properties of spun bonded nonwovens may be manipulated by changing the denier and basis weight of the fibers, as well as changing the polymer(s) used to create the fibers. Some polymers are stiffer, such as polyesters, and some polymers are more flexible, such as polypropylene and polyethylene. Only recently have polyethylene polymers been created with enough draw down to be made into a fiber. Polypropylene is a common polymer used in spun bonded nonwovens and a spun bonded polypropylene nonwoven web is typically referred to as “SBPP”.
It is also desirable for the fibers in the nonwovens to be used in topsheets for absorptive devices be hydrophilic. Natural cellulose fibers are hydrophilic and have historically been used in topsheets. For example, U.S. Pat. No. 6,548,731 to Mizutani, et al. teaches that relatively short hydrophilic fibers may be interspersed with longer hydrophobic fibers to form a topsheet material, and that hydrophobic synthetic fibers may be used if coated with a surfactant to make them hydrophilic. However, the surfactant will generally wash away when subjected to a large amount of liquid, thereby making the synthetic fibers hydrophobic again.
Although synthetic hydrophilic fibers such as rayon, viscose, acetate and spun nylon exist, these polymer types are generally relatively rigid and stiff, and many are difficult to extrude into thin fibers. Therefore, if such materials are used in fibers for a topsheet, the resulting topsheet would tend to be harsh and uncomfortable to the wearer of the absorptive device.
It is desirable to use less costly spun bonded nonwoven materials in absorptive devices and still deliver the softness of a typical spunlace nonwoven material.